This invention relates to multiple layer sheet materials, especially multiple layer packaging sheet materials which are used to fabricate packages through the formation of heat seals about the periphery of the package. The invention is particularly related to the ability of the packages to tolerate flexure of the package and to tolerate shock stresses generated by impact forces, as when the package is dropped. The preferred sheet materials of the invention include a functional capability to provide barrier to transmission of one or more gases such as oxygen or water vapor.
In forming heat seals in such sheet materials, the amount of heat which is driven through the sheet material to the sealant layer to soften that layer sufficiently for formation of the heat seals, as a secondary and undesirable function, may be sufficiently intense to significantly soften an intermediate vinylidene chloride copolymer layer during the formation of the heat seals.
Packages made with conventional ones of such sheet materials, to the extent the overall thickness of the sheet material is less than about 0.38 mm., tend to be deficient in impact resistance; especially packages where such sheet materials are used as closure lids on formed trays. In such applications, the lid is the weakest member of the package, such that any failure of the package typically occurs in the lid sheet material adjacent the heat seal.
"Softening temperature" as used herein means any determinant and measurable temperature which identifies a condition at which the polymer experiences a change which tends to make it more fluid, and subject to flowing at normal conditions used in forming heat seals. While the DSC melting point is generally intended herein, other tests could equally well be used so long as they are applied equally to all the layers being compared.
It is an object of this invention to provide improved multiple layer sheet materials wherein there is provided an impact layer having elastomeric properties and an improved impact tolerance in the resulting sheet material, such that the impact tolerance of the package is improved.
It is especially an object to provide increased impact tolerance at the heat seal locus, while maintaining the ability, in the sheet materials, to form heat seals sufficiently strong to maintain the integrity of the seal throughout the intended use life of the package until it is intentionally opened.